Question

A certain alcohol contains only three elements, carbon, hydrogen, and oxygen. Combustion of a 20.00 gram sample of the alcohol produced 38.20 grams of CO2 and 23.48 grams of H2O. What is the empirical formula of the alcohol?

Answer 1

The empirical formula is C₂H₆O.

We must calculate the masses of C, H, and O from the masses given.

Mass of C =38.20 g CO₂ × (12.01 g C/44.01 g CO₂) = 10.424 g C

Mass of H = 23.48 g H₂O × (2.016 g H/18.02 g H₂O) = 2.6268 g H

Mass of O = Mass of compound - Mass of C - Mass of H

= (20.00 – 10.424 – 2.6268) g = 6.9487 g

Now, we must convert these masses to moles and find their ratios.

From here on, I like to summarize the calculations in a table.

Element  Mass/g    Moles    Ratio  Integers  

     C        10.424    0.8680  1.999        2

     H         2.6268  2.606    6.001        6

     O        6.9487  0.4343    1                1

The empirical formula is C₂H₆O.